I would first check that all the vents are blowing heated air. In some cases the ducting will come loose or not be properly attached at the furnace, therefore the heat does not blow thru the duct and is trapped and recirculated through the furnace return.
If you have the appropriate amount of heated air flow from each vent, yet the trailer will not stay warm I would check for air leaks around the slideouts that may allow cold air in. Also, check in the cabinets where waterlines may go thru the flooring for air leaks.
If air is coming out of vents warm, but at a low rate of flow, you may need to check for kinks in the ducting. You should be able to access the ducting through the return air vent. It will either open like a cabinet door or may have 4 screws to remove.
You didn't say that the air from the furnace was cold, but if that is the case you have issues with the furnace that need to be addressed.